The invention relates to the field of metal recovery from solutions, particularly the recovery of zinc metal from geothermal brines.
The term xe2x80x9cbrines,xe2x80x9d when used in its broad sense, includes brines from a variety of sources, such as geothermal vents, oil drilling, metal mining, and industrial waste and by-product streams. Such brines frequently contain high concentrations of commercially valuable metal ions.
Geothermal brines are of particular interest as they can be used as a source of power for electricity production. In areas such as California, subterranean pools of geothermal brines can be reached by drilling. When piped to the surface, the highly pressurized and heated brines xe2x80x9cflashxe2x80x9d to produce steam, which can be used to run steam turbines and produce electricity. The xe2x80x9cflashedxe2x80x9d brine that results from power production is frequently rich in commercially valuable metals such as zinc, silver, manganese, lithium and lead. Recovery of dissolved metals from flashed steam used for power generation is attractive as a sidestream or downstream process, and has the potential to increase the overall profitability of geothermal power operations.
Methods have been proposed for the recovery of metals from flashed geothermal brines, such as that disclosed in U.S. Pat. No. 4,624,704. This patent discloses the use of a quaternary amine (e.g., a methyl trialkyl ammonium chloride in which the alkyl group is 8 to 10 carbon atoms) dissolved in a water immiscible organic solvent for use in a single step liquid extraction. The loaded solvent extractant is stripped by contacting it with an aqueous sodium sulfate solution, resulting in an aqueous zinc chloride/sodium sulfate phase and an immiscible zinc depleted organic extractant phase.
Other methods rely on precipitation of metals from geothermal brines. For example, Schultze et al. (Geothermal Resources Council, Transactions, 6: 111-113, October, 1982) compare lime precipitation with sulfide precipitation of metals from geothermal brines. This disclosure teaches that precipitation methods can be quite efficient (up to 95% of iron, manganese, lead and zinc can be precipitate by the addition of lime). However, precipitation methods also require that each particular metal be separated from co-precipitated materials. Addition of lime also increases the pH of the brine to levels which make reinjection of the spent brine into the source geothermal formation difficult.
U.S. Pat. No. 5,229,003 discloses a method for zinc recovery from geothermal brines that is of particular relevance. This patent discloses the use of a two-step extract process which may utilize either ion exchange (IX) or solvent (SX) extraction steps. While the method disclosed in the ""003 patent is minimally operable, it suffers from several deficiencies that severely reduce its industrial applicability. For example, this method is susceptible to iron and calcium fouling on and in the IX resin, which reduces the effectiveness and lifespan of the IX resin as well as the profitability of the metal recovery process. The lifespan and effectiveness of the IX resin are critical to the feasibility of a metal recovery operation because IX resins are costly when used in large amounts in metal recovery operations.
Accordingly, there is a need in the art for a method of recovering zinc from zinc-containing brines at high efficiency and low cost.
The invention is directed towards a process for the recovery of substantially pure zinc (Zn) from brines which contain recoverable amounts of Zn and other metals.
In one embodiment, brine is passed over an ion exchange (IX) resin which binds zinc, wherein the resin has been pre-equilibrated with a solution containing a reducing agent. The loaded resin is washed with a hot aqueous acidic xe2x80x98salt washxe2x80x99, then eluted with an aqueous acidic solution containing a reducing agent. The metals deleterious to downstream processes (e.g., iron, arsenic and lead) are removed from the zinc-rich eluate by oxidation followed by solids removal. The zinc is extracted from the eluate in a solvent extraction (SX) process utilizing a water immiscible cationic extractant (SX extractant). After removal of the aqueous (Zn-depleted) phase, the Zn-loaded SX extractant is scrubbed with a dilute acid solution, washed with reverse osmosis (RO) water and then stripped with a sulfuric acid solution. The sulfuric acid/zinc solution is separated from the stripped SX extractant and is used for electrowinning of Zn.
In another embodiment, reducing agent is added to the brine prior to loading onto the IX resin.
In a further embodiment, the loaded IX resin is backwashed prior to elution.
In another embodiment, the SX organic phase is washed with 6 molar (M) HCl prior to remove accumulated iron from the organic. This iron, if uncontrolled, seriously affects the performance of the organic.